Optical component and method for producing the same

ABSTRACT

A concave lens as an optical component is made of glass containing 20 to 22% of B 2 O 3 , 30 to 40% of La 2 O 3 , and 19 to 25% of ZnO, expressed as wt %. The concave lens has a thickness t 1  in its center portion of 0.5 mm or less, and a ratio (W/t 1 ) of a diameter W with respect to the thickness t 1  of 24 or more. The concave lens can be produced suitably by press forming.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical component and a method for producing the optical component.

2. Description of Related Art

Glass lenses to be used for DSC (digital still camera) or DVC (digital video camera) are produced by press forming. Glass lenses having an aspherical shape can be produced at low cost by press forming.

In order to produce lenses with high shape accuracy, it is important to prevent glass from fusion bonding to a forming die, and the forming die is required to have very high surface accuracy and low surface roughness, in press forming. The following techniques are proposed for improving the glass (formed lens) separation from a forming die. JP 62 (1987)-207728 A discloses a technique in which the surface of a glass substrate is provided with a surface layer having a lower concentration of easily volatile components than the glass main body so that high surface accuracy and low surface roughness of a forming die can be ensured. JP 8 (1996)-217468 A discloses a technique for preventing glass from fusion bonding to a forming die by forming a carbon film on the surface of a glass preform.

It should be noted that the term “fusion bonding” in this description means a phenomenon in which a part of glass adheres to a forming die so as to remain as a residue on the surface of the forming die, or a phenomenon in which the formed product (lens) of glass sticks to the forming die. In forming a thin lens, either phenomenon can occur.

In recent years, the demand for reduction in size of image pickup optical systems represented by DSC and DVC has been increasing more and more. In order to reduce the size of image pickup optical systems, it is necessary to reduce the lens thickness. Specifically, it is necessary to establish a technique for mass producing very thin lenses such as concave lenses having a thickness of 0.5 mm or less in its center portion and a convex lens having a thickness of 0.5 mm or less in its outer peripheral portion (edge portion).

However, it is very difficult to produce such a thin lens by conventional press forming methods. In forming a lens, load is concentrated on the thin portion of the lens, thus causing glass to be fusion bonded to a forming die notably. The fusion bonding of glass to the forming die not only deteriorates the shape accuracy of a lens as a finished product, but also impairs the surface accuracy of the forming die because of the residual glass adhering to the forming die, which makes it difficult to use the same forming die continuously. Although the techniques disclosed in JP 62 (1987)-207728 A and JP 8 (1996)-217468 A have some effect in preventing the glass from fusion bonding to the forming die, the effect is still insufficient. Further, the techniques of JP 62 (1987)-207728 A and JP 8 (1996)-217468 A suffer from problems such as an increase in the number of steps, and an increase in cost, because various pretreatments are required therein.

SUMMARY OF THE INVENTION

It is an object of the present invention to shed light on a glass composition suitable for a thin lens, and to provide a lens (optical component) produced using the glass.

That is, the present invention provides an optical component made of glass containing 20 to 22% of B₂O₃, 30 to 40% of La₂O₃, and 19 to 25% of ZnO, expressed as wt %. The optical component is formed as a concave lens that has a thickness t₁ in its center portion of 0.5 mm or less and a ratio (W/t₁) of a diameter W with respect to the thickness t₁ of 24 or more, or a convex lens that has a maximum thickness in its outer peripheral portion of 0.5 mm or less.

According to another aspect of the present invention, there is provided a method for producing an optical component including the steps of: supplying a preformed body made of glass for an optical component into a forming die; closing the forming die; transferring the surface shape of the forming die to the preformed body by heating and pressing the preformed body; and opening the forming die in order to remove the formed optical component therefrom. The glass for the optical component contains 20 to 22% of B₂O₃, 30 to 40% of La₂O₃, and 19 to 25% of ZnO, expressed as wt %, and the optical component is formed as a concave lens that has a thickness t₁ in its center portion of 0.5 mm or less and a ratio (W/t₁) of a diameter W with respect to the thickness t₁ of the center portion of 24 or more, or a convex lens that has a maximum thickness in its outer peripheral portion of 0.5 mm or less.

The inventors have found that there is a correlation between the likelihood of the occurrence of glass fusion bonding to a forming die and the composition of the glass. That is, use of the glass containing components within the range as mentioned above can prevent the glass from fusion bonding to a forming die, so that stable production of thin lenses by press forming is rendered possible. The present invention eliminates the need for particular pretreatments also, and thus is free from the problems such as an increase in the number of steps and an increase in cost. The lens provided by the present invention enables a reduction in the size of image pickup optical systems such as DSC.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top view and a sectional view illustrating an optical component (concave lens) according to one embodiment of the present invention.

FIG. 2 shows a top view and a sectional view illustrating an optical component (convex lens) according to another embodiment of the present invention.

FIGS. 3A to 3D are views illustrating the steps of the method for producing a lens.

DETAILED DESCRIPTION OF THE INVENTION

As indicated in FIG. 1, a lens 1 according to one embodiment of the present invention is formed as a concave lens having a concave surface 1 a and a convex surface 1 b. The lens 1 is a so-called aspherical lens and has a meniscus shape. The lens having a meniscus shape means a crescent shaped lens with one surface being convex and the other surface being concave. Types of meniscus lenses include a concave meniscus lens in which the outer peripheral portion has a thickness larger than the center portion, and a convex meniscus lens in which the center portion has a thickness larger than the outer peripheral portion. The lens 1 indicated in FIG. 1 is a concave meniscus lens.

The lens 1 is suitable for small optical devices, and has a diameter W, for example, in the range of 10 to 20 mm. The center portion of the lens 1 has a thickness t₁, for example, in the range of 0.1 to 0.5 mm. The ratio (W/t₁) of the diameter W with respect to the thickness t₁ of the center portion is, for example, 24 or more. The upper limit of the ratio (W/t₁) is not limited particularly, but for example, is 200.

As indicated in FIG. 2, a lens 2 according to another embodiment of the present invention is formed as a biconvex lens having two convex surfaces 2 a and 2 b. The lens 2 also is a so-called aspherical lens. The lens 2 further has an outer peripheral portion 2 c (edge portion) with a reduced thickness. The outer peripheral portion 2 c is a portion that is used for fixing the lens 2 to a housing in an optical device, and that does not form an optical path. As is the lens 1, the lens 2 also is suitable for small optical devices, and has a diameter W, for example, in the range of 10 to 20 mm. The outer peripheral portion 2 c of the lens 2 has a maximum thickness t₂, for example, in the range of 0.1 to 0.5 mm.

Next, the method for producing the lens 1 is described. The production method according to this embodiment employs a press forming method (so-called precision glass forming) using a forming die. First, a preformed body to be used for producing the lens 1 is prepared. Specifically, the preformed body is obtained by grinding and/or polishing optical glass with a particular composition to process it into an analogous shape to the lens 1.

In the case of producing a thin lens by press forming, it is important to prevent glass from fusion bonding to the forming die. As seen from the later mentioned examples, there is a correlation between the likelihood of the occurrence of the glass fusion bonding to the forming die and the composition of the glass. That is, by using the glass containing 20 to 22% of B₂O₃, 30 to 40% of La₂O₃, and 19 to 25% of ZnO, expressed as wt %, it is possible to prevent the fusion bonding of the glass to the forming die effectively.

Further, the glass to be used for producing the lens 1 (or the lens 2) has a refractive index (nd) in the range of 1.79 to 1.83 and an Abbe number (νd) in the range of 39 to 43, for example. The glass having a high refractive index is advantageous in reducing the thickness of the lens. The glass having a high Abbe number enables clear images to be obtained.

A detailed example of the glass composition that satisfies the requirements mentioned above is described below. The values are expressed as wt %.

SiO₂: 3 to 6%, preferably 5 to 6%

B₂O₃: 20 to 22%, preferably 20 to 21%

Li₂O: 0 to 2%, preferably 0 to 1%

ZnO: 19 to 25%, preferably 20 to 24%

ZrO₂: 3 to 5%, preferably 3 to 4%

TiO₂: 0 to 2%, preferably 0 to 1%

Nb₂O₅: 2 to 7%, preferably 5 to 7%

La₂O₃: 30 to 40%, preferably 34 to 40%

Ta₂O₃: 0 to 10%, preferably 0 to 4%

WO₃: 0 to 7%, preferably 0 to 2%

Next, as indicated in FIGS. 3A to 3D, a preformed body 10 is press formed. A forming device provided with an upper die 3, a lower die 4, a cylindrical die 5, an upper head 6 and a lower head 7 can be used for press forming. In the forming device, the upper head 6 is provided above the upper die 3, and the lower head 7 is provided so as to support the lower die 4. The upper head 6 is provided with heating and pressing mechanisms (not shown in the drawings). Similarly, the lower head 7 is provided with a heating mechanism.

The upper die 3 includes a convex portion 3 a and a supporting portion 3 b that surrounds the convex portion 3 a. The convex portion 3 a is a portion in contact with the concave surface of the preformed body 10, and the supporting portion 3 b is a circular portion in contact with the outer peripheral portion of the preformed body 10. Each surface of the upper die 3 and the lower die 4 is processed to have a desired accuracy and shape so that the lens 1 to be shaped by the upper die 3, the lower die 4 and the cylindrical die 5 has optical properties as designed. Depending on the circumstances, the surface of the cylindrical die 5 also is processed to have a desired accuracy and shape.

The upper die 3, the lower die 4 and the cylindrical die 5 typically are made of cemented carbide such as WC. The cemented carbide is preferable as a material of forming dies because of its excellent strength. The upper die 3, the lower die 4 and the cylindrical die 5 each may be constituted by a base material made of metal such as stainless steel and a plating layer (for example, electroless nickel plating layer) covering the surface of the base material. Furthermore, a releasing layer may be formed on each surface of the upper die 3, the lower die 4 and the cylindrical die 5 for improving the releasability and the corrosion resistance.

First, the preformed body 10 is supplied into a forming die that is constituted by the upper die 3, the lower die 4 and the cylindrical die 5 as indicated in FIG. 3A.

Next, the upper head 6 is lowered, so that the preformed body 10 is clamped by the upper die 3, the lower die 4 and the cylindrical die 5 as indicated in FIG. 3B. Then, the heating mechanisms in the upper head 6 and the lower head 7 are activated in this state to heat the preformed body 10 to a predetermined temperature. The predetermined temperature is, for example, a slightly higher temperature than the glass transition temperature (Tg) or the yield temperature (At) of the glass to be used (for example, a temperature higher than the Tg or At by 20 to 40° C.). Preferably, the heating is performed at an approximate temperature increase rate in which the preformed body 10 takes several minutes to reach the predetermined temperature (for example, 5 minutes).

After the preformed body 10 has reached the predetermined temperature, the upper head 6 is lowered slowly and thereby pressure is applied on the upper die 3, as indicated in FIG. 3C. The pressure from the upper head 6 is applied on the preformed body 10 via the upper die 3, so that the preformed body 10 is deformed gradually. Thus, the shape of each surface of the upper die 3 and the lower die 4 is transferred to the preformed body 10. The pressure to be applied on the preformed body 10 is, for example, 50 to 300 kgf.

When the preformed body 10 is deformed into the shape of the lens 1, the upper head 6 stops applying the pressure. Then, this state is maintained at the above-mentioned predetermined temperature for about 1 minute. Thereafter, the heating mechanisms in the upper head 6 and the lower head 7 are turned off, and the lens 1 is cooled to the glass transition temperature while the upper head 6 is kept lowered. In order to avoid the occurrence of defects in the lens 1 such as distortion, it is preferable that the lens 1 be cooled slowly over several minutes (for example, about 5 minutes).

Finally, the upper head 6 and the upper die 3 are opened so that the formed lens 1 can be removed therefrom, as indicated in FIG. 3D. It should be noted that each step of FIGS. 3A to 3D can be performed under an inert atmosphere such as a nitrogen atmosphere or an argon atmosphere. Such shape forming under an inert atmosphere is preferable for extending the life span of a forming die because the oxidation of the forming die can be prevented.

The concave meniscus lens 1 described with reference to FIG. 1 generally has the concave surface 1 a that is inclined more steeply than conventional lenses such as a spherical lens and convex meniscus lens. Therefore, the glass is likely to be fusion bonded to a forming die. Accordingly, as is this embodiment, it is particularly effective to employ glass having a composition that is unlikely to cause the occurrence of fusion bonding as a material of the concave meniscus lens. Of course, such glass is significant also for a biconvex lens that is a lens having two convex surfaces in combination (FIG. 2), a biconcave lens that is a lens having two concave surfaces in combination, and a planoconvex lens and planoconcave lens each being a lens with one flat surface.

EXAMPLES

Using glass having a composition and optical constant indicated in Table 1, concave meniscus lenses having a shape indicated in FIG. 1 were produced by press forming that has been described with reference to FIGS. 3A to 3D. Specifically, using glass No. 1 to 3, 20 lenses for each of 4 kinds of concave meniscus lenses that have a thickness t₁ in the center portion, respectively, of 0.9 mm, 0.7 mm, 0.5 mm and 0.3 mm were produced continuously. The presence or absence of fusion bonding of the glass to the forming die was investigated. All the lenses were set to have a diameter W of 14 mm, an approximate concave curvature of 4 mm, and an approximate convex curvature of 280 mm. The press forming process was performed under a nitrogen atmosphere. At the time of glass formation, the heating temperature was set to 600° C., and the pressure force was set to 150 kgf. Table 1 indicates the results.

TABLE 1 GLASS No. 1 GLASS No. 2 GLASS No. 3 nd 1.81 1.81 1.81 νd 41 41 41 SiO₂ 5.4 3.5 3.2 B₂O₃ 20.4 20.8 18.8 Li₂O 0.6 1.5 2.0 ZnO 22.6 20.9 19.7 ZrO₂ 3.3 3.2 4.2 TiO₂ 0.8 — — Nb₂O₅ 6.2 2.6 2.0 La₂O₃ 39.2 34.0 25.2 Gd₂O₃ — — 10.3 Ta₂O₃ — 9.9 7.9 WO₃ 1.6 3.7 6.7 FORMATION EVALUATION 0.9 mm ∘ ∘ ∘ 0.7 mm ∘ ∘ x 0.5 mm ∘ ∘ x 0.3 mm ∘ Δ x

In Table 1, the formation was evaluated as “o” that means no fusion bonding occurred, “Δ” that means fusion bonding occurred in 1 to 3 lenses, and “x” that means fusion bonding occurred in 4 or more lenses, when 20 lenses were produced continuously.

In the case of using the glass No. 1, even if the thickness of the center portion was reduced to 0.5 mm or less, the glass was not fusion bonded to the forming die, and stable formation of concave meniscus lenses having a desired shape was possible. In the case of using the glass No. 2, slight fusion bonding occurred when the center portion of the lens had a thickness of 0.3 mm, though favorable results were obtained in general. In contrast, in the case of using the glass No. 3, the glass was fusion bonded to the forming die in a significant percentage when the thickness of the center portion was 0.7 mm or less, and stable formation was not feasible.

As a result of X-ray photoelectron spectral analysis (XPS) for the surface of each forming die after concave meniscus lenses were formed using the glass No. 1 to No. 3, peaks based on B, La and Zn were observed. That is, B, La and Zn contained in the glass were present on the surface of the forming die. The present amount of B, La and Zn was larger in the order of the forming die used for the glass No. 1, the forming die used for the glass No. 2, and the forming die used for the glass No. 3. In view of these, it is conceivable that when the glass contains an appropriate amount of B, La and Zn, these components cause a releasing effect by spreading or diffusing on the surface of the forming die, thereby rendering the fusion bonding of the glass to the forming die unlikely to occur.

From the above results, it can be said that use of glass that contains 20 to 22% of B₂O₃, 30 to 40% of La₂O₃, 19 to 25% of ZnO, expressed as wt %, can prevent the glass from fusion bonding to a forming die. The preferable contents of B₂O₃, La₂O₃, and ZnO are respectively 20 to 21%, 34 to 40%, and 20 to 24%. It may seem that the greater the contained amounts of these components, the more the prevention effect of the fusion bonding is enhanced, but when they are contained in an excess amount, there are problems such as the difficulty of achieving a desired optical constant and the possibility of causing the devitrification of glass. Specifically, an excess content of B₂O₃ makes it difficult to adjust the refractive index to a desired range. An excess content of La₂O₃ causes glass devitrification to occur easily. An excess content of ZnO makes it difficult to adjust the refractive index and Abbe number to a desired range. Accordingly, each component should be contained within the above-mentioned range. By using the glass with such a composition, concave lenses that have a thickness t₁ in its center portion of 0.5 mm or less and a ratio (W/t₁) of a diameter W with respect to the thickness t₁ of the center portion of 24 or more can be produced by press forming at a high yield. By preventing the glass fusion bonding to a forming die, the surface accuracy of the forming die also can be kept high and the surface roughness thereof can be kept low. This can reduce the frequency of maintenance for a forming die, and as a result, it becomes possible to achieve high productivity.

It should be noted that tendencies similar to the case of the concave lens were observed in the case of forming the convex lens indicated in FIG. 2. That is, although the thinner the outer peripheral portion 2 c was, the more the fusion bonding of glass to a forming die occurred easily, stable formation of convex lenses having a maximum thickness t₂ in its outer peripheral portion 2 c of 0.5 mm or less was possible by using the glass No. 1 and No. 2.

The optical component (optical lens) of the present invention can be used suitably for optical devices such as DSC, DVC, cell phone cameras, projection televisions, optical pickups, and the like.

The present invention may be embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments described in this specification are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. 

1. An optical component made of glass containing 20 to 22% of B₂O₃, 30 to 40% of La₂O₃, and 19 to 25% of ZnO, expressed as wt %, the optical component being formed as a concave lens that has a thickness t₁ in its center portion of 0.5 mm or less and a ratio (W/t₁) of a diameter W with respect to the thickness t₁ of 24 or more, or a convex lens that has a maximum thickness in its outer peripheral portion of 0.5 mm or less.
 2. The optical component according to claim 1, wherein the glass has a refractive index (nd) in the range of 1.79 to 1.83, and an Abbe number (νd) in the range of 39 to
 43. 3. The optical component according to claim 1, wherein the optical component is produced by press forming.
 4. A method for producing an optical component comprising the steps of supplying a preformed body made of glass for an optical component into a forming die; clamping the preformed body with the forming die; transferring a surface shape of the forming die to the preformed body by heating and pressing the preformed body; and opening the forming die in order to remove the formed optical component therefrom, wherein the glass contains 20 to 22% of B₂O₃, 30 to 40% of La₂O₃, and 19 to 25% of ZnO, expressed as wt %, and the optical component is formed as a concave lens that has a thickness t₁ in its center portion of 0.5 mm or less and a ratio (W/t₁) of a diameter W with respect to the thickness t₁ of 24 or more, or a convex lens that has a maximum thickness in its outer peripheral portion of 0.5 mm or less. 